Printing apparatus of toner jet type having an electrically screened matrix unit

ABSTRACT

A printing apparatus includes heat treatment element, a rotatable feeder roll chargeable to a predetermined first potential, a support roll chargeable to a predetermined second potential, and a matrix in the form of a flexible printing circuit. The matrix has supply apertures, each supply aperture having a first inner diameter and being surrounded by an electrically conducting control ring configured to be charged to a predetermined third potential and having a second inner diameter. The third potential is selected to control corresponding supply apertures between an open state and a closed state. The matrix has an upper surface which is covered with a protective layer having through holes, each through hole having a second diameter which is at least equal to the inner diameter of the control rings. The protective layer includes a non-magnetic metal. The matrix and the control rings are covered, at upper surfaces and bore edges, with an electrically insulating layer. The feeder roll, the support roll and the matrix are configured to transfer a dry powder from the feeder roll through the supply apertures of the matrix to an object to be printed which is conveyed over the support roll. The powder deposited on the object is fixed by the heat treatment element.

FIELD OF THE INVENTION

The present invention generally relates to a printing apparatus of the ktype which is used in various types of printers, for copying machines,in telefacsimile machines etc., and which operates with a dry printpowder which is, in electrical way, applied to the object to be printed,for instance the paper, and which is thereafter fixed to the paper,generally by a heat treatment.

SUMMARY OF THE INVENTION

The invention is more particularly directed to a printing apparatus ofsaid type which named a “toner jet” printing apparatus, and in which adry print powder, generally named “toner”, is, by a direct method,transferred from a rotating toner feeder roll, through bores of a fixedmatrix in the form of a flexible printing circuit and down onto theobject to be printed, for instance the paper which is conveyed over asupport roll and in which the print powder which has been applied to thepaper is finally fixed to the paper by a heat treatment.

The basis of said process is that two electrical fields are created fortransferring toner from the toner feeder roll to the paper, namely afirst electrical field between the toner feeder roll and the tonermatrix, which electrical field is brought to invert polarity, and asecond, preferably constantly downwards directed positive electricalfield between the matrix and the support roll above which the paper istransferred.

The toner matrix is formed with a large number of very small throughbores having a diameter of for instance 100-300 μm, and round eachindividual such bore an electrically conducting ring of a suitablemetal, like copper, in the following referred to as “copper ring”. Eachcopper ring can be charged with a positive potential, for instance+300V, which is higher than the potential of the toner feeder roll,which potential can for instance be between +5V and +100V, preferablyabout +50V, but which is less than the potential of the support roll forthe paper, which potential can for instance be +1500V. When theelectrically conducting ring is charged with a voltage said ring makesthe belonging matrix bore become “opened” for letting toner down. If thematrix bore ring is, on the contrary, charged with a potential which issubstantially lower than the potential of the toner feeder roll, forinstance in that said ring is connected to ground, the belonging matrixbore becomes “closed” thereby preventing a letting down of toner.

The function is as follows:

the toner powder gets a negative potential in that said toner particlesrub against each other;

the toner powder is supplied to the toner feeder roll, which ispositively charged by a predetermined potential, often a potential whichcan be controlled between +0V and +100V, and the toner powder isdistributed in an even, sufficiently thick layer over the toner feederroll using a doctor blade;

each bore of the matrix which corresponds to a desired toner dot isopened in that the matrix bore ring is charged with a positive potentialwhich is higher than the potential of the toner feeder roll, forinstance +300V; bores corresponding to non toner carrying portionsremain connected to the ground, whereby said bores are to be considered“closed”, thereby making it impossible to let toner through; thecombination of opened matrix bores forms the image to be reproduced;

depending on the difference in potential, for instance +50V to+300V=250V between the toner feeder roll and the toner matrix negativelycharged toner particles are sucked down from the toner feeder roll tothe matrix, and depending on the difference in potential between thetoner matrix and the support roll mounted underneath same, for instance+300V to +1500V=+1200V the toner particles are moved on from the matrixand deposit on the paper above the support roll;

the paper with toner deposited thereon is finally moved through a heattreatment apparatus in which the toner is fixed to the paper.

There is an almost linear relationship between the density of thecurrent field and the traction force that said field exerts on the tonerparticles. The field has its greatest density just above the copperrings, and the density decreases from the ring edges towards the centerof the bore. By reducing the potential of the toner feeder roll, whichleads to an increasing difference in potential between the toner feederroll and the matrix, it is possible to increase the amount of tonerwhich is let down. An increase of the potential of the toner feeder roilleads a corresponding reduction of the amount of toner which is letdown.

By connecting the copper ring of the matrix to the ground the directionof potential between the toner feeder roll is reversed from having been+250V in the direction downwards to be +50V in the direction upwards,and this makes negatively charged toner particles stick to the tonerfeeder roll, or be sucked back thereto, respectively.

In a certain embodiment of the printing apparatus the distance betweenthe toner feeder roll and the matrix was about 0.1 mm, and the distancebetween the matrix and the support roll was about 0.6 mm. At normalprinting the toner feeder roll has a voltage of +50V, and this gives adifference in potential to the matrix, which can have a voltage of+300V, of +250V between the toner feeder roll and the matrix. over theabove mentioned distance of 0.1 mm this gives a field strength of2.5V/μm.

The distance between the toner feeder roll and the support roll is about0.7 μm, and the difference in potential is +1450V. This gives a fieldstrength of 2 V/μm between the bottom surface of the matrix and thepaper. The same electric field is present above the matrix and betweenthe copper rings, and said field acts against the toner on the tonerfeeder roll, so that toner particles can be released from the tonerfeeder roll and can fall down on the upper surface of the matrix. Assoon as the toner particles reach a copper ring, which is connected toground (0V), said toner particles jump back to the toner feeder roll,and after having passed the copper ring said particles jump back down tothe matrix again.

It also can happen that toner which is present above a conduit to acopper ring when the voltage changes from 0V to +300V can be sucked tothe upper surface of the matrix and can be kept thereon, and this canprevent other toner particles from being fed into the matrix bore at thecentre of the copper ring.

Toner which jumps up and down between the toner feeder roll and theupper surface of the matrix obstacles the flow of toner past theprinting zone, and the jumping toner particles are often unloaded or mayeven change charge to the non-desired positive charge. Also, a slightamount of the toner particles normally have a “false” potential,generally 2-4% of the toner particles, and such falsely charged tonerparticles are often sucked both to the upper surface and to the bottomsurface of the matrix.

SUMMARY OF THE INVENTION

The present invention is intended to solve the problem that tonerparticles jump between the toner feeder roll and the matrix, and saidproblem is solved in that a thin, protective metal layer is applied onthe upper surface of the matrix. Said protective layer is formed withbores the diameters of which coincide with the outer diameter of thecopper rings. The layer is given the same potential as that of the tonerfeeder roll, for instance +50V. The protective layer can have athickness of 20-30 μm, and it is glued onto the upper surface of thematrix. The protective metal layer acts as an electric screen betweenthe toner feeder roll and the matrix with the electric conduits thereof.

It is important that the bores of the protective layer each have adiameter which is at least the same as the outer diameter of the copperrings, since there would otherwise be a risque that the layer mightscreen off the field between the toner feeder roll and copper rings. Inorder to prevent that the material between the bores of the protectivelayer is too narrow the matrix is preferably formed with the copperrings on the top of the matrix base and with the inner diameter of thecopper ring the same size as that of the bores of the matrix, wherebythe copper rings may be used to a maximum for feeding toner particlesfrom the toner feeder roll, through the matrix and down to the paper. Ina matrix having a toner feeder bore with a diameter of about 190 μm thecopper rings can have an outer diameter of for instance 250 μm, and insuch case the bores of the protective layer can preferably be given adiameter of 250 μm.

If the toner feeder roll and the toner is of magnetic type theprotective layer has to be of an unmagnetic material like of stainlesssteel, beryllium copper, hard nickel, brass, aluminum or another hard,unmagnetic material.

In order to eliminate the risque of flash over between the toner feederroll and the matrix and between the copper rings and the support roll itis therefore necessary that the matrix bore ring be insulated. This isdone in that the entire matrix is covered, for instance by anevaporation process, with an insulating substance which encloses allfree surfaces and edges of the matrix, the matrix bores and theprotective layer. An available method is the method named the Parylene®method (Union Carbide) according to which a polymeric insulationmaterial named poly-para-xylene, using a vacuum apparatus, is applied tothe matrix in a very well predetermined thickness. The material has anelectric decomposition resistance of about 200 V/μm. This means that itis sufficient to use a layer having a thickness of only 2 μm forinsulating an electric field of +250V between the toner feeder roll andthe copper ring of the matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically and in a perspective view the basic principleof a printing apparatus of toner-jet type.

FIG. 2 shows schematically in enlarged scale a cross section viewthrough a printing apparatus of toner-jet type according to the priorart.

FIG. 3 shows a cross section view through a printing apparatus accordingto the present invention.

FIG. 4 shows in an enlarged scale the part of FIG. 3 which is encircledby a broken circle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Thus, in FIG. 1 there is diagrammatically shown a printing apparatus oftoner jet type comprising a toner feeder roll 1 having an outer layer 2of a toner powder of known type, a toner matrix 3 mounted underneath thetoner feeder roll 1 and a support roll 4 mounted underneath the matrix 3which support roll is arranged to support an object to be printed whichis conveyed between the matrix 3 and the support roll 4, which object isnormally a paper 5.

In FIG. 2 is diagrammatically shown that some toner particles can bereleased from the toner feeder roll 1 and may deposit as waste toner 2 aat the upper surface of the matrix 3. Such waste toner obstacles afeeding down of toner particles through the toner feeder bores of thematrix. In some cases waste toner also may deposit on the bottom surfaceof the matrix, and such toner may smear off on the paper 5 as anon-desired back ground tone.

As shown in FIG. 3 a toner container 6 is mounted above the rotatabletoner feeder roll 1, and from said container 6 toner is let down on thetoner feeder roll 1. A doctor blade 7 spreads and distributes the tonerto form an even layer 2 of toner on the toner feeder roll 1. The tonerfeeder roll 1 is charged with a certain positive voltage of for instancebetween +5 and +100V, in the illustrated case a voltage of about +50V.Since the toner particles rub against each other they are charged with anegative polarity, and this makes the toner particles become adhered tothe positively charged toner feeder roll.

The matrix 3 is formed with a large number of through bores 8 adapted tolet toner through when said bores are in open condition. Said bores canhave a diameter of 100-300 μm, in a certain tested matrix a diameter of190 μm. Round each toner bore 8 there is an electrically conducting ring9, for instance of copper, for controlling the letting through of tonerparticles. For enabling a maximum letting down of toner through thetoner bores 8 the copper ring is mounted on top of the matrix with itsinner diameter flush with the toner bore 8. Each copper ring 9, orcontrol ring, is over conduits 10 electrically connected to a controlmeans 11 which is diagrammatically shown in FIG. 3 and which is arrangedto alternatively charge the copper ring either with a voltage which ishigher than the voltage of the toner feeder roll 1, for instance avoltage of +300V, whereby the matrix bore is “opened”, or with a voltagewhich is lower than the voltage of the toner feeder roll, in particulara voltage of ±0V, in that the ring is connected to ground, whereby thematrix bore is “closed”.

The opening of the toner matrix bore 8 is thus accomplished in that thecopper ring 9 is given a potential of for instance +300V, whereby adifference in potential of +300-+50=+250V appears between the tonerfeeder roll 1 and the matrix 3. Said difference in potential is so greatthat the negatively charged toner particles are released from the tonerfeeder roll 1 and are sucked down against the matrix 3 and through thepresently opened matrix bores 8. If the copper ring 9 is connected toground the direction of potential is inversed and there appears anupwardly directed difference in potential of +50V, and toner particlesare thereby sucked back towards the toner feeder roll 1, or are keptthereon, respectively. As mentioned above toner particles may, however,be released from the toner feeder roll 1 and deposit on the matrix, ormay jump up and down between the toner feeder roll 1 and the matrix 3.

The support roll 4 constantly has a voltage which is higher than thehighest voltage, +300V, of the matrix 3, in the illustrated case avoltage of +1500V. In “opened” matrix bores 8 there is consequently adownwards directed difference in potential of +1200V, and saiddifference makes toner particles become sucked down from the matrix 3towards the support roll 4. Toner particles deposit as dots on the paper5 which is moved over the support roll 4. A series of such dots fromseveral matrix bores successively form the image or images to berepresented on the paper.

The paper with the toner particles deposited thereon is thereafterpassed through a heat treatment apparatus, for instance between twoheater rolls 12, in which the toner powder is fixed to the paper.

The distances between the different parts marked in the drawings are,for the sake of clearness, strongly exaggerated. The distance betweenthe toner feeder roll 1 and the matrix 3 can, for instance, be 0.1 mmand the distance between the matrix 3 and the support roll 4 can, forinstance, be 0.6 mm.

As indicated with the dotted lines in FIG. 3 the matrix 3 may preferablybe bowed in a curvature the axis of which coincides with the axis ofrotation for the toner feeder roll 1. For further stabilising the matrix3 and avoiding vibrations which may bring the bottom surface of thematrix 3 in contact with the paper 5 the bottom surface of the matrix 3can be laminated with a (not illustrated) metal layer, which ispreferably also enclosed in an insulating layer.

For avoiding flash over between the toner feeder roll 1 and the matrix 3and between the matrix 3 and the support roll 4 the copper rings 9 ontop of the matrix 3 have to be insulated. The insulation is accomplishedin that the electrically conducting copper rings 9 are connected, in asuitable way, to the upper surface of the matrix base 11, for instanceby means of glue or tape, so that the copper ring 9 with the innerdiameter thereof is flush with matrix bore 8. Thereafter the entirematrix 3 is covered with a thin layer 14 of an insulation material whichcovers the entire matrix at the top surface and the bottom surface andalso extends over the inner edges both of the matrix bores 8 and thecopper rings 9. Such covering can be accomplished by an evaporationprocess with an insulation substance, whereby said substance enclosesall free surfaces of the matrix, the matrix bores and the copper rings.An available method is named the Parylene® method (Union Carbide)according to which process a polymeric insulation material namedpoly-para-xylene is, in a vacuum apparatus, applied to the matrix in avery accurately controlled layer thickness. The material has an electricdecomposition resistance of about 200V/μm. This means that it issufficient with a thickness of the insulation layer 14 of only 2 μm forinsulating an electric field of 250V between the toner feeder roll andthe copper ring of the matrix. For the sake of safety the material isgenerally applied in a layer having a thickness of 5-10 μm. Even usingsuch great thickness of the insulating layer as 10 μm for a matrix bore8 having a diameter of 170 μm and an inner diameter of the copper ring 9of 190 μm the specific opening area for the matrix bore 8 for lettingtoner through is as great as 89,9%. This provides a great margin inprinting with the printing apparatus in that a more even print qualitycan be obtained. At the same time problems depending on variations inmoisture and temperature are reduced. It is also possible, thanks to theincrease in degree of blackness during the printing, to reduce the drivevoltage of the control rings 9 and to increase the tolerances of certainparts included in the apparatus.

For eliminating the problem that toner particles are released from thetoner feeder roll 1 and deposit on the upper surface of the matrix 3,and in some cases also the bottom surface of the matrix, or that tonerjumps up and down between the toner feeder roll 1 and the matrix 3 thereis provided a protective layer 15 of metal on top of the matrix. Theprotective layer must be made of an unmagnetic metal and can be ofstainless steel, beryllium copper, hard nickel, brass, aluminum oranother hard, unmagnetic material. The protective layer 15 is formedwith through bores 16 equivalent to the bores 8 of the matrix and thebores of the copper rings 9. For foreseeing that the protective metallayer 15 does not provide an electric screen against the copper rings 9the bores 16 of the protective layer 15 preferably should be at least aslarge as the outer diameter of the copper rings 9. The protective layer15 is, via a conduit, charged with the same voltage as that of the tonerfeeder roll, in the illustrated case a voltage of +50V. Since the tonerfeeder roll 1 and the protective metal layer 15 has the same voltage andpolarity there is no electric field between said parts, and there isconsequently no force tending to tear off toner particles from the tonerfeeder roll. For the same reason it is also not necessary to provide anyinsulation of the protective metal layer 15.

REFERENCE NUMERALS 1 toner feeder roll 2 toner layer 3 toner matrix 4support roll 5 paper 6 toner container 7 doctor blade 8 toner feederbore 9 copper ring 10 conduit (for 9) 11 control means 12 heater rolls13 matrix base 14 insulation layer 15 protection layer 16 bore 17conduit

What is claimed is:
 1. A printing apparatus comprising: heat treatmentmeans; a rotatable feeder roll chargeable to predetermined firstpotential; a support roll chargeable to a predetermined secondpotential; and a matrix in the form of a flexable printing circuit, saidmatrix having feeder bores, each feeder bore having a first diameter andbeing surrounded by an electrically conducting control ring configuredto be charged to a predetermined third potential and having an innerdiameter, said third potential being selected to control correspondingfeeder bores between an open state and a closed state, said open statebeing achieved when said third potential is higher than said firstpotential and lower than said second potential, and said closed statebeing achieved when said third potential is lower than said firstpotential, said matrix having an upper surface which is covered with aprotective layer having through holes, each though hole having a seconddiameter which is at least equal to the inner diameter of the controlrings, said protective layer being of a non-magnetic metal and chargedwith a voltage which is substantially equal to the first potential ofthe feeder roll, said matrix and said control rings being covered, atupper surfaces and bore edges, with an electrically insulating layer;wherein said feeder roll, said support roll and said matrix areconfigured to transfer a dry powder from said feeder roll through saidfeeder bores the matrix to an object to be printed which is conveyedover said support roll, said powder deposited on the object being fixedby said heat treatment means.
 2. The printing apparatus of claim 1,wherein the metallic protective layer includes a hard metal selectedfrom a group including stainless steel, beryllium copper, hard nickel,brass and aluminum.
 3. The printing apparatus of claim 1, wherein theinner diameter of each control ring of the matrix is substantially equalto the first diameter of the feeder bore of the matrix.
 4. The printingapparatus of claim 3, wherein each electrically conducting control ringis secured directly on top of the matrix with the inner diameter of thecontrol ring flush with the bore of the matrix.
 5. The printingapparatus of claim 4, wherein the electrically insulating layer is alayer of a polymeric material having a predetermined thickness.
 6. Theprinting apparatus of claim 5, wherein the polymeric material includespoly-para-xylene.
 7. The printing apparatus of claim 4, wherein theinsulating material of the matrix is applied by an evaporation method.8. The printing apparatus of claim 1, wherein the matrix is bent in acurvature, said curvature having an axis which coincides with an axis ofrotation of the feeder roll, and wherein the matrix has a stabilizingmetal layer at the surface facing the object to be printed.
 9. Theprinting apparatus of claim 1, wherein the inner diameter of eachcontrol ring of the matrix is substantially equal to the first diameterof the feeder bore of the matrix.
 10. The printing apparatus of claim 9,wherein each electrically conducting control ring is secured directly ontop of the matrix with the inner diameter of the control ring flush withthe bore of the matrix.
 11. The printing apparatus of claim 10, whereinthe electrically insulating layer is a layer of a polymeric materialhaving a predetermined thickness.
 12. The printing apparatus of claim11, wherein the polymeric material includes poly-para-xylene.
 13. Aprinting apparatus comprising: heat treatment means; a rotatable feederroll chargeable to a predetermined first potential; a support rollchargeable to a predetermine second potential; and a matrix in the formof a flexible printing circuit, said matrix having feeder bores, eachfeeder bore having a first diameter and being surrounded by anelectrically conducting control ring configured to be charged to apredetermined third potential and having an inner diameter, the innerdiameter of each control ring of the matrix being substantially equal tothe first diameter of the feeder bore of the matrix, each electricallyconducting control ring being secured directly on top of the matrix withthe inner diameter of the control ring flush with the bore of thematrix, said third potential being selected to control correspondingfeeder bores between an open state and a closed state, said open statebeing achieved when said third potential is higher than said firstpotential and lower than said second potential, and said closed statebeing achieved when said third potential is lower than said firstpotential, said matrix having an upper surface which is covered with aprotective layer having through holes, each through hole having a seconddiameter which is at least equal to the inner diameter of the controlrings, said protective layer being of a non-magnetic metal, said matrixand said control rings being covered, at upper surfaces and bore edges,with an electrically insulating layer, wherein the electricallyinsulating layer has an electrical decomposition resistance of about 200V/μm, and wherein said layer is applied in a thickness of more than 2 μmfor insulating an electric field of +250 V between the feeder roll andthe control ring of the matrix; wherein said feeder roll, said supportroll and said matrix are configured to transfer a dry powder from saidfeeder roll through said feeder bores the matrix to an object to beprinted which is conveyed over said support roll, said powder depositedon the object being fixed by said heat treatment means.
 14. The printingapparatus of claim 13, wherein the thickness is about 5-10 μm.
 15. Aprinting apparatus comprising: heat treatment means; a rotatable feederroll chargeable to a predetermined first potential; a support rollchargeable to a predetermined second potential; and a matrix in the formof a flexible printing circuit, said matrix having feeder bores, eachfeeder bore having a first diameter and being surrounded by anelectrically conducting control ring configured to be charged to apredetermined third potential and having an inner diameter, said thirdpotential being selected to control corresponding feeder bores betweenan open state and a closed state, said open state being achieved whensaid third potential is higher than said first potential and lower thansaid second potential, and said closed state being achieved when saidthird potential is lower than said first potential, said matrix havingan upper surface which is covered with a protective layer having throughholes, each through hole having a second diameter which is at leastequal to the inner diameter of the control rings, said protective layerbeing of a non-magnetic hard metal selected from a group includingstainless steel, beryllium copper, hard nickel, brass and aluminum, saidprotective layer being charged with a voltage which is substantiallyequal to the first potential of the feeder roll, said matrix and saidcontrol rings being covered, at upper surfaces and bore edges, with anelectrically insulating layer; wherein said feeder roll, said supportroll and said matrix are configured to transfer a dry powder from saidfeeder roll through said feeder bores the matrix to an object to beprinted which is conveyed over said support roll, said powder depositedon the object being fixed by said heat treatment means.
 16. The printingapparatus of claim 15, wherein the inner diameter of each control ringof the matrix is substantially equal to the first diameter of the feederbore of the matrix.
 17. The printing apparatus of claim 6, wherein eachelectrically conducting control ring is secured directly on top of thematrix with the inner diameter of the control ring flush with the boreof the matrix.
 18. The printing apparatus of claim 17, wherein theelectrically insulating layer is a layer of a polymeric material havinga predetermined thickness.